Typical VDF and its model. Panel a: slice of the measured VDF in a plane containing B, with parallel and perpendicular axis centered on the solar wind velocity. Panel b: reconstructed VDF. Panels c, d: plots of the distribution along the perpendicular and the parallel axis. The initial distribution is in black, the core bi-Maxwellian in red, the remaining population (initial minus core) in green, and the beam model in blue. The VDF unit is s³ m⁻⁶. The maximal VDF value is 2.5 10¹⁰.